A stretch borrowed from yoga, modified slightly. A personal favourite of mine, if I've been standing or sitting for long periods of time.
It "lengthens" your spine, stretches the lower back and "lifts" the mid-back. Particularly useful if you have a "deep" lordosis (pronounced backward arch in the lower back) or a strong kyphosis (tendency to "slouch" or bend forward in the mid-back), which can sometimes happen if you spend a lot of time at a computer.
Pay attention to the placement of your elbows, as this is the key to success with this one.